CPU Frequency

On the LoPy, I'd like to set the CPU frequency to 8 Mhz to see if it has an effect on power consumption. I am building and deploying the firmware, using the source code here: https://github.com/pycom/pycom-micropython. I can't easily find the place in the code where the 16 Mhz clock speed is currently being set.

Does anyone know what to change to bring the clock down to 8 Mhz.

Are there any other optimizations folks know that I can do to bring the power consumption down? Currently, with WiFi and BT off, the power consumption is ~50 mA. When awake, this adds up, and I'd like to get it down to about 20 mA.